json-datasources like maximo-datasources can be added to, removed from, updated, etc. The important thing to understand is that a json-datasource by default is only in memory and won't be sent back to the Maximo server. The use cases out of the box load data into the JSON datasources to temporarily display data and then changes are fetched from the JSON datasource and sent via a maximo-datasource.
It's an advanced topic but there is the concept of a custom-datasource where you can implement your own data-adapter to retrieve, send, etc. from another system (doesn't have to be Maximo).
Most use cases are better if you stick with a maximo-datasource. This avoids taking data out of a maximo-datasource to put into a json-datasource, implement the schema (so Maximo can understand data types, lengths, etc.), and then capture the changes from the json-datasource to put back into the maximo-datasource. There were use cases out of the box (in particular, the inventor apps) that used to use JSON datasources and moved back to maximo-datasource to improve performance. There are legitimate scenarios to use a json-datasource (like the allowable status list is a great example of where this makes sense) but generally, you won't need to use a json-datasource.
Regarding saved queries, those are out of the box all on the object structures in Maximo. If you open the Integration Objects application, filter for the object structure mentioned, and then use the Query Definition action to open the dialog. But as a FYI, this is NOT a requirement. You can use existing queries from existing applications (like WOTRACK) with the REST API.
You can modify these queries but be aware that the product could too in a patch and your change would be lost. You can also add your own queries if it's transactional or a new lookup data. Avoid creating a new query for existing lookup datasource. There's a page in the Developer Documentation that covers the issue. Look at Mobile Developers->Overview in that wiki to understand the limitation.